The speed at which electrical signals can travel limits the speed at which data can be transferred. One solution is to convert the transfer of signals from the transfer of electrons to the transfer of photons which are capable of carrying high speed, high data rate computing signals (hundreds of MHz to GHz frequencies). These interconnects may be chip-to-chip, as in an optical multichip module, or connect single packaged chips on a printed circuit board.
Optical data transfer can be accomplished by an optical waveguide having a transparent "optical" core guiding material that is embedded in a cladding material. The optical signal is transmitted through the core material via total internal reflection. Optical waveguides are used at the printed circuit board level for clock distribution and interconnection of single chip packages and on silicon substrates for interchip connection at the multichip module level.
One requirement for the cladding material is that the refractive index of the cladding material be less than the refractive index of the core material. For passive guides, the cladding could be air, but polymer claddings are typically preferred so that the core material is isolated from any conducting (metallization) layers.
Useful optical waveguides must have low optical transmission loss, low optical absorbance, and controllable refractive index.
Another useful property is high thermal stability, which is necessary so that the waveguide will survive electronic packaging and assembly processes used in manufacturing. The optical multichip module would have to survive semiconductor assembly processes such as die attach, metallization, and wire bonding. The printed circuit board would have to survive reflow soldering and rework.
A particularly useful property is that either the core or the cladding materials can be photodefined into channels or ridges with smooth sidewalls using UV exposure techniques.
Still another useful property is that multilayer structures can be formed by overcoating one polymer layer over another. In other words, the first layer of the multilayer structure would be resistant to the solvent used in the subsequent "overcoat" layer.
Polymethylmethacrylate (PMMA) is one photodefinable polymer that has been used for optical waveguides. However, PMMA has low thermal stability and cannot be used at the high temperatures needed for most electronic applications, for example, greater than 300.degree. C. needed for die attach and soldering procedures used in the manufacturing of printed circuit boards.
Photodefinable polyimides are typically not used as waveguides because many conventional photodefinable polyimides are not transparent. Further, although polyimides are known to have the thermal stability required for electronic and semiconductor applications, many semiconductor grade polyimides display a high optical absorbance in the near IR visible region. Since typical commercial laser and light sources emit in the near IR visible range (350 nm to 2,000 nm) a polymer having a high optical absorbance in this region is generally not desirable for use as a photodefinable waveguide.
EP 454,590 discusses low optical loss waveguides that are made from isotropic polyimides. The reference teaches that 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl is a necessary component of the invention. However, the polymers are not photosensitive. Further, an extensive multilayer resist process and an extensive dry etching process is taught in the examples as necessary to pattern these polymers.
Further, the European reference lists BTDA, among many dianhydrides, as one possible component of their waveguides. However, there are no examples teaching the use of BTDA and there is no recognition that BTDA must be carefully balanced to provide a useful photosensitive material for use as a waveguide. BTDA has a very strong absorbance maximum at 310 nm to 330 nm. Therefore, polyimide compositions incorporating large amounts of BTDA would be expected to have a very high optical absorbance loss, a property which is not preferred in an optical waveguide.
The EP 454,590 reference also teaches that the homopolyimides of their invention cannot be multilayered unless the first layer has been heat treated (cured) at a temperature of not less than 380.degree. C. This high temperature heat treatment is probably necessary because highly fluorinated polyimides are known to have a low solvent resistance. The polyimides of the present invention, however, are copolyimides that are photosensitive and that can be photochemically crosslinked. Crosslinking is believed to improve solvent resistance so that the polyimides of the present invention can be multilayered after a cure temperature of only 350.degree. C.
U.S. Pat. No. 4,912,197 discloses 6FDA/BTDA/DMDE polyimides. The polyimides are highly soluble, clear compositions that are photochemically crosslinkable. The reference does not address photosensitivity or photodefinability. However, the 6FDA/BTDA/DMDE polyimides have a high birefringence which causes them to scatter light (see Comparative Examples A-C).
The polyimides of the present invention provide a photodefinable polyimide that incorporates a co-diamine moiety which contributes to reduced birefringence. Birefringence is a measure of the optical anisotropy (i.e., wherein the properties are different in one direction, for example in-plane, than they are in another direction, for example, out-of-plane). The typical rigid rod polyimides that are generally preferred for semiconductor applications are highly oriented in the plane of the coating and thus exhibit a high birefringence. High birefringence measurements correspond to high optical scattering losses and irreproducibility of the refractive index.
Further, it is unexpected that the incorporation of a non-photocrosslinkable co-diamine in the polyimide of the present invention would result in a polyimide having high photosensitivity since the introduction of a co-diamine should reduce the concentration of the photocrosslinkable group in DMDE.
Moyer, E., PhD Thesis, Virginia Polytech Institute, 1989, (page 168) found that the reduction of the methyl substituted diamines greatly decreased photosensitivity. When Moyer replaced DMDE with 40 mol % of a co-diamine 1,3-bis(3-aminophenoxy-4'-benzoyl) benzene (DKEDA), his photosensitivity dropped from 188 mJ/cm.sup.2 to 1,388 mJ/cm.sup.2. However, we have discovered that certain co-diamines will provide a polyimide having reduced birefringence and that still retains high photosensitivity.
U.S. Pat. No. 4,657,832 discloses photosensitive polyimides where the diamine is substituted with alkyl groups but the disclosed polyimides do not encompass the use of the 6F dianhydride. Further, this reference does not teach the use of a co-diamine to reduce birefringence in the polyimide.
NASA Technical Support Package LAR-13539 and U.S. Pat. Nos. 4,595,548 and 4,603,061 disclose transparent aromatic polyimides derived from various dianhydrides, including 6FDA, and ether or thioether bridged diamines. However, these compositions are not photosensitive. Furthermore, the NASA Package teaches the use of aryl ether diamines such as OBA and BDAF to reduce charge transfer complexes and increase transparency. We have shown that OBA and BDAF containing polyimides are not sufficiently photosensitive for use in the waveguides of the present invention (see Comparative Examples D and F).
U.S. Pat. No. 4,705,540 discloses the use of 6FDA/DMDE as a gas permeable membrane. The disclosed compositions do not include BTDA and are not photosensitive. Also, this reference teaches the use of rigid diamines and co-diamines with hindered rotation. The rigid diamines would be expected to increase birefringence making the polymer unsuitable for waveguides.
U.S. Pat. No. 4,717,393 discloses auto-photochemically crosslinked gas separation membranes. The reference does not teach the use of a co-diamine to decrease optical loss.
The present invention provides an optical waveguide made from a thermally stable, low optical loss, low optical absorbance polyimide. The refractive index of the polyimides of the present invention can be controlled by changing the composition of the polyimide (for example, compare Example 1 to Example 7) or by substituting one co-diamine for another (compare Example 1 to Example 3).
The polyimides of the present invention are photosensitive and can be photodefined which affords their fabrication into the waveguide structures of the present invention by UV exposure. Further, the polyimides of the present invention can be etched or channeled by wet etch techniques.
The polyimides of the present invention are solvent resistant and can be fabricated into multilayer structures by overcoating one polyimide layer over another.
In addition, we have found that fluorinated co-diamines reduce absorbance at 1.3 microns, an important wavelength for telecommunications. Therefore, in the present invention, fluorinated co-diamines are preferred.